Compression techniques are commonly used in digital facsimile systems for minimizing the time required for transmission of documents. Source coding--a technique which reduces the redundancy inherent in the input document--is the most common technique used for this purpose. Examples of source coding methods include the Modified Huffman code and the modified READ code. A fundamental characteristic of source coding is that, assuming no transmission errors, the output document produced is similar to the input digital image, i.e., no approximations or degradation occurs relative to the original digitized image. Another characteristic of source coding is that the compression ratio provided is highly dependent upon the complexity of the input document so that complex documents require substantially more transmission time than simple documents containing little information.
A further prior art technique of particular interest here is that disclosed in U.S. Pat. No. 4,193,092 (Stoffel). The Stoffel patent discloses a image interpolation system wherein analog data pixels are sampled and held so as to produce a "box car" wave representative of the image data. Differences in wave height are bridged by linear interpolation, with frequency enhancement being imposed at the junctures of the bridges. The modified wave is processed to provide an image pixel stream comprised of both original image pixels and one or more interpolated image pixels therebetween. One or more intermediate image lines are interpolated by comparing binary image data to produce an address used to identify a specific image pixel pattern from multiple image patterns stored in a look-up table in memory. The image pixel pattern identified is used to form the interpolated line or lines. A further patent of interest in this respect is U.S. Pat. No. 3,956,578 (de Loye et al) which relates to compression techniques for facsimile systems for picture transmission.
Other patents of more general interest include U.S. Pat. Nos. 3,347,981 (Kagan et al); 3,804,975 (Abe); 3,950,609 (Tanaka et al); 4,086,620 (Bowen et al); 4,096,527 (Furata); and 4,316,222 (Subramanian).